The present invention generally relates to the art of gantry cranes and more particularly to a gantry crane having a hydraulically actuated hoist system.
Gantry cranes are conventionally used in railyards, shipping yards, and other places for loading and unloading containers and loads to and from railcars, trailers, pallets, etc. Such a crane typically has a rigid steel frame that holds a grappler in an elevated manner. More particularly, the frame defines a wide central opening to span across or straddle a truck or railcar while suspending the movable grappler overhead. Typically, the frame includes a plurality of vertical beams mounted to a plurality of horizontal overhead beams. Such cranes are mobile, having a plurality of wheels on which the crane can be driven around a loading yard.
To movably support a grappler, a common crane design includes a pair of movable horizontal stabilizer beams. Each of these stabilizer beams extends between two of the vertical beams of the frame, the stabilizer beams being mounted for vertically slidable movement. The grappler is mounted to the stabilizer beam for lifting containers or other loads.
The crane includes a hoist system for vertically moving the stabilizer. In particular, the hoist system includes a hoist actuator operable to drive chains, cables or other hoist lines for vertically moving the stabilizer beams and the grappler. Through controlling the hoist actuator, the grappler can be raised or lowered to engage container. For example, the hoist actuator may be linear actuator such as a cylinder or a ball screw mechanism, which may be hydraulic, pneumatic, electric, etc.
Unfortunately, hoist actuators have been known to "drift" or slowly move under the load of the grappler over a period of time when the crane is parked. For example, in an embodiment wherein the hoist actuator is a hydraulic cylinder or motor, some degree of internal or external leakage can occur, particularly after seals become worn during service. If the hydraulic cylinder "drifts" due to internal leakage, the grappler slowly lowers toward the ground. The drifting movement is usually so slow as to be imperceptible over a short period to a human observer. This drifting can, however, be problematic.
In particular, the drifting of a grappler is dangerous if the crane has been parked to straddle a live railroad track or road. Cranes are often parked in this manner in a loading yard due to convenience or lack of space. Although an operator may have initially parked the crane with the grappler at a raised position to clear the travel path below, the hoist system may drift over time, causing the grappler to lower. An accident can occur if the grappler undesirably lowers into the pathway of a train or truck passing through the frame of the crane.
Accordingly, a need exists for a means to prevent drifting of the hoist system. More particularly, a need exists for locking the grappler in a raised position indefinitely.